Java 8 groupingby with returning multiple field - java-8

Question

In Java 8 group by how to groupby on a single field which returns more than one field. In the below code by I am passing name and the field to be summed which is 'total' in this scenario. however I would like to return sum of 'total' and 'balance' field for every 'name' in the Customer list (can be a map with key and value as array).
Can it be done by using a single groupingBy with the return values?
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;
public class Sample {
public static void main(String str[]){
Customer custa = new Customer("A",1000,1500);
Customer custa1 = new Customer("A",2000,2500);
Customer custb = new Customer("B",3000,3500);
Customer custc = new Customer("C",4000,4500);
Customer custa2 = new Customer("A",1500,2500);
List<Customer> listCust = new ArrayList<>();
listCust.add(custa);
listCust.add(custa1);
listCust.add(custb);
listCust.add(custc);
listCust.add(custa2);
Map<String, Double> retObj =
listCust.stream().collect(Collectors.groupingBy(Customer::getName,Collectors.summingDouble(Customer::getTotal)));
System.out.println(retObj);
}
private static class Customer {
private String name;
private double total;
private double balance;
public Customer(String name, double total, double balance) {
super();
this.name = name;
this.total = total;
this.balance = balance;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getTotal() {
return total;
}
public void setTotal(double total) {
this.total = total;
}
public double getBalance() {
return balance;
}
public void setBalance(double balance) {
this.balance = balance;
}
#Override
public String toString() {
return "Customer [name=" + name + ", total=" + total + ", balance=" + balance + "]";
}
}
}
Expected Output -
{
A = [4500,6500],
B = [3000,3500] ,
C = [4000,4500]
}

Answer 1

You can write your own collector to sum total and balance
Collector<Customer, List<Double>, List<Double>> collector = Collector.of(
() -> Arrays.asList(0.0, 0.0),
(a, t) -> {
a.set(0, a.get(0) + t.getTotal());
a.set(1, a.get(1) + t.getBalance());
},
(a, b) -> {
a.set(0, a.get(0) + b.get(0));
a.set(1, a.get(1) + b.get(1));
return a;
}
);
Map<String, List<Double>> retObj = listCust
.stream()
.collect(Collectors.groupingBy(Customer::getName, collector));
System.out.println(retObj);
result
{A=[4500.0, 6500.0], B=[3000.0, 3500.0], C=[4000.0, 4500.0]}

Answer 2

You can also use the toMap collector to accomplish the task at hand.
Map<String, List<Double>> retObj =
listCust.stream()
.collect(Collectors.toMap(Customer::getName,
c -> new ArrayList<>(Arrays.asList(c.getTotal(), c.getBalance())),
(l, l1) -> {
l.set(0, l.get(0) + l1.get(0));
l.set(1, l.get(1) + l1.get(1));
return l;
}));